The electrodeless lamps with which the present invention is concerned generally comprise a lamp envelope containing a plasma-forming medium. To operate the lamps, the medium in the envelope is excited with microwave, R.F. or other electromagnetic energy, thereby generating a plasma, which emits radiation in the ultraviolet, visible or infrared part of the spectrum.
It is known that electrodeless lamps transfer a great deal of heat to the envelopes during operation, and it has been found that the effectiveness with which the lamp envelopes may be cooled is a limiting factor in overall lamp performance. Thus, the brightness with which energy is radiated by the lamp increases with the power density of the microwave or other energy in the lamp envelope, but as the power density increases, so does envelope temperature, with a point being reached where the envelope melts if it is not adequately cooled. Thus, the brightness which can be obtained from the lamp is ultimately a function of cooling. Also, in the case where a lamp is operating satisfactorily at a given envelope temperature, cooling the envelope further has the effect of substantially increasing bulb lifetime.
An improved method and apparatus for cooling electrodeless lamps is described in U.S. Pat. No. 4,485,332 issued Nov. 27, 1984 to Ury, et al. for "Method and Apparatus for Cooling Lamps". As disclosed therein, apparatus is provided for rotating the lamp envelope while a stream of cooling gas is passed over its outer surface. The lamp envelope is rotated by an electric motor which has its shaft secured to an elongated stem which is attached to the lamp envelope. The outer end of the elongated stem is provided with a metal ferrule which is cemented into a coupling. The coupling in turn is secured to the motor shaft as by a set screw or a ball detent arrangement.
Some improvements for electrodeless lamps require a reduction in the overall length of the distance between the motor and the lamp envelope. However, the length of the ferrulecoupling shaft combination cannot be reduced to a substantial extent.